A flow sensor for measuring the quantity of air sucked in internal combustion engines is proposed in various types, however, that in service actually comes in, for example, a moving vane system, a Karman's vortex system and a hot wire (or film) system specified in SAE Paper 811428.
Then, where the rate of fuel fed into internal combustion engines is to be measured by means of such flow sensor, it is preferable that an air measuring sensor and a fuel measuring sensor be used in the same type from the industrial viewpoint.
However, in the case of the above-mentioned moving vane system and Karman's vortex system, it is difficult to dispose a detecting unit in a fuel passage for its being large-sized, which is not to meet feasibility.
On the contrary, the hot wire (or film) system is capable of having the detecting unit being small-sized disposed in the fuel passage.
However, when the hot wire (or film) system is employed as air measuring sensor and fuel measuring sensor, it is found that since fluids to be measured are different each other, a detection output is given in an erroneous value from connecting a detection circuit to which the air measuring sensor is connected straight to the fuel measuring sensor.